You are using both modal toggling methods: via Javascript and via data attributes. So your click is firing the modal show as your data attributes set, and your Javascript does not affect this trigger.
Just remove the data attributes and go with the Javascript method:
<button class="button primary" id="buy" style="text-decoration:none;" type="button">Review and confirm</button>
<div class="modal-bootstrap fade bs-example-modal-sm" id="myModal" tabindex="-1" role="dialog" aria-labelledby="smallModalLabel" aria-hidden="true">
<!-- modal contents -->
</div>
<script type="text/javascript">
$("#buy").click(function () {
var a = 4;
if (a == 5) {
alert("if");
$('#myModal').modal('show');
} else {
alert("else");
$('#myModal').modal('hide');
}
});
</script>
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